Quantum-counting x-ray detectors, referred to also as photon-counting x-ray detectors, have as a rule an arrangement of a plurality of detector elements (pixels) composed of a directly converting semiconductor material. A detected radiation quantum generates in the respective detector element a charge pulse which is converted by the detector's electronic circuitry into a measuring voltage which in one or more comparators is compared with threshold voltages representing different energy levels. A detected photon can in that way be assigned a specific energy and counted accordingly.
Because the size of the charge pulse or, as the case may be, the measuring voltage generated therefrom depends on the energy of the incident radiation quantum, the radiation quanta that are counted can be spectrally selected via the setting chosen for the electric threshold height or, as the case may be, the threshold value of the comparator. The only radiation quanta that will be counted are those which owing to their energy produce an electric signal exceeding the comparator's threshold value.
For counting the incident x-ray quanta on an energy-resolved basis the detector needs to be energy calibrated. Calibrating has to be performed separately for each detector element or, as the case may be, measuring channel in order to take account in each case of the specific behavior of the detector material and signal-processing electronic circuitry. Setting the threshold voltages as precisely as possible is crucial to achieving a homogeneous response behavior on the part of the detector and hence, for example when the detector is employed in computed tomography, to obtaining CT images that are as artifact-free as possible and exhibit low drift. For energy calibrating quantum-counting x-ray detectors it is possible to use, for example, radioactive compounds, synchrotron-light sources, or K-fluorescence radiation sources that emit defined spectral lines or quanta. Each threshold value of the comparator is assigned an energy threshold as the result of energy calibrating.
Because some of the sources suitable for calibrating pose problems with handling and are not readily available, an option is to employ fluorescence calibrating using K-fluorescence radiators that are not subject to such limitations. Fluorescence calibrating has, though, hitherto been performed in a laboratory, with the x-ray detectors being illuminated exclusively by way of x-ray fluorescence radiation. The x-ray flux then occurring is very slight and accordingly not matched to a scenario applying to a clinical scanner. Because, however, the detectors (sensor material or, as the case may be, detector elements and signal-processing electronic circuitry or, as the case may be, ASICs) assume a different operating point when being irradiated, the laboratory calibrations are not optimally matched to the conditions applying to use in the x-ray installation so that what results is a shift in the threshold values.